The main objective of the proposed research is to understand the mechanism by which a visual pigment molecule both excites and light adapts a photoreceptor. Our approach is to quantitatively investigate adaption and its spatial localization within both vertebrate and invertebrate photoreceptors. We also use quantitative studies of adaptation and its spatial localization as tools for investigating hypotheses about the mechanisms of visual excitation and adaptation. One of our immediate goals is to assess the role of Ca ions in visual excitation and adaptation.